The present invention relates to ternary alloys of aluminium and titanium.
A common approach to design of high strength aluminium alloys for elevated temperature applications involves production of alloy microstructures comprising a large volume fraction of finely and homogeneously dispersed, thermally stable intermetallic particles. Those alloying elements favoured in such developments are those miscible with aluminium in the liquid state and having low solid solubilities and diffusivities in the solid state, for these will contribute to low coarsening rates of the strengthening dispersoids. Similarly, the preferred intermetallic phases are those which are intrinsically stable at elevated temperatures and which possess low interfacial energy in an aluminiummatrix. The production of suitable microstructures commonly involves rapid solidification processing, during which the solubility of alloying elements may be increased and a large volume fraction of fine-scale dispersoids may be generated either directly from the melt during rapid quenching, or from supersaturated solid solution by suitable post-solidification heat treatments.
The most successful group of alloys developed using this approach has been that based on the Al-Fe system, with ternary and often quaternary additions. However, in these alloys, the dispersed intermetallic phases mostly form directly from the melt during rapid solidification and are relatively coarse in scale. The alloys themselves are of relatively high density, since to achieve the required volume fractions of dispersed phases requires large concentrations (8-12 wt %) of what are commonly higher density solute elements (Fe, Mo, V, Zr, Cr, Ce). There thus remains scope for the design and development of improved alloys, particularly alloys of lower density, refined microstructure and improved thermal stability.
Of the possible alternatives, the Al-Ti system appears one of the most promising. The titanium is low density and has low solid solubility and diffusivity in aluminium. Under conditions of rapid solidification, formation of the equilibrium intermetallic phase Al.sub.3 Ti (b.c.t., DO.sub.22) is generally suppressed and replaced by a metastable ordered cubic (Ll.sub.2) phase, that is sub-stoichiometric with respect to titanium (.about.Al.sub.4 Ti). The metastable intermetallic particles have a cube-cube orientation relationship and a low lattice misfit with the matrix phase, and would thus be expected to possess a low interfacial energy. However, in binary Al-Ti alloys, the metastable intermetallic phase forms directly from the melt, and is thus relatively coarse in scale (0.1-0.3 .mu.m). In addition, the volume fraction of fine-scale, solid state intermetallic precipitates is low in quenched and aged microstructures and the distribution is inhomogeneous [6,7]. Limited attempts have been made to refine microstructures by ternary alloying additions to rapidly quenched dilute Al-Ti binary alloys. However, detailed analysis of the rapidly quenched microstructures and the microstructural evolution during post-solidification ageing treatments has received little attention.
An object of the present invention is to provide ternary alloys of aluminium and titanium of high strength.